1. Field of the Invention
This invention relates to a fluid pressure brake control apparatus for a vehicle which controls braking slip and driving slip of wheels.
2. Description of the Prior Art
For example, Japanese Patent Opening Gazette No. 134361/1987 discloses a braking slip control apparatus or lock preventing apparatus. In this apparatus, a fluid pressure control valve is arranged between the master cylinder and the wheel brake apparatus. When the brake to the wheel brake apparatus is decreased, the brake fluid is discharged into the hydraulic reservoir through the fluid pressure control valve from the wheel brake apparatus. And when the brake to the wheel brake apparatus is again increased, the brake fluid discharged into the hydraulic reservoir is pressurized by a fluid pressure pump and supplied into the conduit between the fluid pressure control valve and the wheel brake apparatus.
On the other hand, necessity of a driving slip control apparatus together with a braking slip control apparatus has arisen in order to suppress the excessive slip of the drive wheel on the rapid start and the rapid acceleration with the rapid rise of the engine power and to facilitate the driving operation and improve the steering ability, the acceleration and rapid starting ability of the vehicle.
In order to solve the above described problem, this Applicant improved the apparatus described in the above described Japanese Patent Opening Gazette No 134361/1987. In this improved apparatus (Japanese Utility Model Registration Application No. 54240/1988), a reservoir for a tandem master cylinder is used commonly as the hydraulic reservoir connected to the fluid pressure control valve. Thus, when the brake to the wheel brake apparatus is decreased, the brake fluid is discharged into the reservoir for the tandem master cylinder, and the brake is applied in the driving slip control by the fluid pressure pump. The discharging fluid of the fluid pressure pump is returned into the reservoir for the tandem master cylinder in the brake decreasing operation. The return conduit is arranged to by-pass the input and output sides of the fluid pressure control valve. In order to prevent the brake application to a wheel brake apparatus from being impossible, a cut-off valve is arranged in the return conduit.
However, if the brake fluid discharged directly into the reservoir for the master cylinder is not returned through the conduit connecting the fluid pressure control valve with the master cylinder into the fluid pressure generating chamber side of the master cylinder, after the driving slip control ends, the piston stroke of the master cylinder would be unnecessarily large or the stroke of the brake pedal would become unnecessarily long.
After the brake fluid pressure to the wheel brake apparatus does not need to be controlled, the amount of the brake fluid discharged should be returned to the fluid pressure generating chamber of the master cylinder by the fluid pressure pump. Therefore, the control operation is complicated, and to infer the amount of the discharged brake fluid is very difficult. For example, a fluid level switch may to be arranged in the hydraulic reservoir to measure the discharged amount of brake fluid. However, this method is inaccurate and the construction is complicated. Further, the apparatus has a defect that the kick-back phenomena to the brake pedal occur.